Oscar Pistorius said he was left "heartbroken" at discovering he had acciodentally shot dead his girlfriend Reeva Steenkamp.
The murder accused runner said he had been screaming in panic after it dawned on him it was Steenkamp who had been in the toilet and not an intruder.
He was then struck dumb with grief after breaking down the door with a cricket bat to see the model, aged 29, lying in a pool of blood.
Prosecutor Gerrie Nel claimed the screams were Steenkamp's as she fled from him to hide in the toilet.
During the final stages of gruelling interrogation by Nel, Pistorius was asked why he stopped screaming when he saw what he had done.
The 27-year-old replied: "I was heartbroken [...] overcome with sadness."
Nel wrapped up his cross-examination by hammering home his argument that the disabled star gunned down Steenkamp after an argument between them.
Again accusing Pistorius of failing to accept responsibility for his actions, Nel asked him: "Who should we blame for the fact you shot her?"
Pistorius gave the same answer which trial observers believe has done harm to his credibility during his spell in the witness stand: "I don't know."
Continuing his point, Nel said the screams described by four witnesses were made by Steenkamp fleeing in terror from Pistorius. He said the runner armed himself "for the sole purpose" of killing the blonde model. Nel then said Reeva had been talking to Pistorius when he opened fire – hitting her three times.
The prosecutor concluded: "Mr Pistorius, I'm putting to you, you killed Reeva and you're the only person who can give us a version."
Pistorius denies premeditated murder and claims he mistook Steenkamp for an intruder inside his luxury home on the outskirts of Pretoria. The prosecution alleges he killed her following an argument between the couple.
He also faces charges of illegally firing a gun in public and of illegally possessing ammunition, both of which he denies.
There are no juries at trials in South Africa, and his fate will ultimately be decided by Judge Thokozile Masipa.
The trial continues.